Electrophotographic imaging systems employ a photoconductor surface that is first charged to a uniform voltage and is then discharged in accordance with an image to be reproduced. A scanned laser beam accomplishes the discharge action. The photoconductor thereby gains an electrostatic latent image that constitutes a matrix of discharged pixel sites. In a black/white printer, the photoconductor surface is generally developed using a black toner that adheres to the discharged pixel sites to form the image. Thereafter, the toned photoconductor is carried to a transfer station where the image is transferred to a media sheet.
In multicolor printers, successive images are developed, employing different color toners that are supplied from corresponding toner modules. The color printing is normally accomplished through use of yellow, cyan and magenta toners that are applied, in registration, either during successive rotations of the photoconductor surface, or during a single rotation of the photoconductor surface.
The toning process is based on electrostatic attraction of charged toner particles to areas of opposite sign charge voltage on the photoconductor. The charge voltage on the photoconductor may be either positive or negative relative to the toner particles. Many electrophotographic printers employ discharge area development wherein the toner is attracted to, and tones, the areas that have been discharged by the scanned laser. Alternatively, a toner may be used which has an opposite sign charge to the photoconductor and results in charge area development. In such a process, the laser discharges the areas that are not intended to receive the toner. The toner, which is of the opposite sign compared to the charged image areas, is electrostatically repelled by the discharge areas and is attracted to the opposite sign charged areas.
In the prior art, imaging systems have employed toners that exhibit multiple charge states to accomplish a multicolor print action. U.S. Pat. No. 5,450,189 to Russell et al., assigned to the same Assignee as this application, discloses an electrostatic imaging system wherein the photoconductor is charged to a first charge potential. Areas of the photoconductor are then discharged to a second charge potential in accordance with applied image signals. A first toner that exhibits a charge state that is attracted by the second (discharge) charge potential, but is repelled by the first charge potential, is utilized to tone the discharged regions on the photoconductor. A second toner is also utilized that exhibits an opposite charge state to the first toner's charge state. Thus, the second toner is attracted by the first charge potential and is repelled by the second charge potential.
A controller enables the first toner to be applied to the imaged photoconductor, with the entire photoconductor thereafter being recharged. Then, non-imaged areas of the photoconductor are discharged to a charge potential that repels the second toner. The second toner is then applied to imaged areas that remain at the first charge potential.
The Russell et al. system requires two charging actions to accomplish the deposition of the plural toners, and further requires toners exhibiting different sense charge states.
Electrophotographic printers that provide multiple gray levels employ halftone arrangements of printed dots to form the gray regions. Such halftoning generally employs single, solid dots to form each pixel. The more levels of gray that are required of the printer, the greater the number of dots needed, per pixel, to accomplish the gray levels.
Current personal and business laser printers exhibit dot resolutions of 1200 dots per inch (dpi). A 1200 dpi laser printer forms 75 pixels per inch, each pixel comprising an 8-bit gray value. Use of a single color black toner to provide multiple gray levels requires, potentially, a substantial number of dots per pixel. There is a need to provide such printers with an ability to produce multiple gray levels using a smaller number of dots to form the required gray levels. Such a smaller number of dots enables higher print speeds to be attained, while retaining the desired high quality print output demanded by consumers.
Accordingly, it is an object of this invention to provide an improved electrophotographic method and system for producing multiple gray level print outputs.
It is another object of this invention to provide an improved electrophotographic print method and system that employs toners exhibiting a single charge sense.
It is yet another object of the invention to provide an improved electrophotographic printer wherein multiple gray levels are provided during a single cycle of operation of the photoconductor surface.